1. Field of the Invention
The present invention relates to a laser processing method, and more particularly to a laser processing method used for cutting and cleaving or division processing of a material to be processed.
2. Description of the Background Art
A laser processing method using laser beam to cut and cleave a material to be processed has been conventionally known (see, for example, Japanese Patent Laying-Open No. 2002-192370 (hereinafter referred to as PTL 1), Japanese Patent Laying-Open No. 2002-205180 (hereinafter referred to as PTL 2) and Japanese Patent Laying-Open No. 2008-6492 (hereinafter referred to as PTL 3)).
PTL 1 discloses that a wafer-like material to be processed is irradiated with laser beam having a pulse width of 1 μs or less such that a focusing point (focus) of the laser beam is arranged inside the material to be processed, thereby forming a modified area, which serves as a starting point of cutting processing, inside the material to be processed. PTL 2 discloses that a wafer-like material to be processed is irradiated with laser beam having a pulse width of 1 μs or less such that a focusing point (focus) of the laser beam is arranged inside the material to be processed, and the position of the focusing point in the incidence direction of the laser beam is changed, thereby forming a plurality of modified areas, which serve as a starting point of cutting processing, in the depth direction inside the material to be processed. PTL 3 discloses that in cutting a sapphire substrate with laser beam, laser beam having an extremely-short pulse width in a femtosecond range is used to minimize damage caused by irradiation with the laser beam.
The aforementioned conventional laser processing methods, however, have had the following problems. Specifically, in the laser processing method disclosed in PTL 1, the irradiated laser beam has a single wavelength and is focused on one point inside the material to be processed. Therefore, the length of the modified area in the thickness direction of the material to be processed cannot be increased too much. As a result, the direction and the like of a crack developing from the modified area in cutting processing are not fixed in some cases, which may result in variations in the shape of a cut surface. In the laser processing method disclosed in PTL 2, although the plurality of modified areas can be formed in the depth direction of the material to be processed, laser beam irradiation must be performed a plurality of times to form the plurality of modified areas, which causes a problem of an increase in the number of steps in the manufacturing process and thus an increase in the manufacturing cost. In the laser processing method disclosed in PTL 3, although the laser beam having an extremely-short pulse width in the femtosecond range is used, the industrial use of such laser beam having an extremely-short pulse width is not practical in terms of the apparatus cost, the stability of the process and the like.
As described above, it has been conventionally difficult to cut the material to be processed by laser processing in a short time and at low cost, and to achieve the excellent shape of the cut surface.